The study compared a low-fat diet to a Mediterranean diet -- a diet rich in whole grains, vegetables, fruits, legumes, nuts, fish and olive oil. The Mediterranean diet was supplemented with either extra nuts or extra virgin olive oil.

The Mediterranean diet didn't lower the odds of developing metabolic syndrome -- a collection of risk factors for heart disease -- compared to following a low-fat diet, the study found. But, the Mediterranean diet did increase the chance of reversing metabolic syndrome, said lead researcher Dr. Jordi Salas-Salvado, a professor of nutrition at the Universitat Rovira I Virgili and Hospital Universitari de Sant Joan de Reus, in Spain.

"It seems that the Mediterranean diet supplemented with nuts or virgin olive oil has similar effects on the metabolic syndrome reversal," he said. Those on the Mediterranean diet with extra olive oil were 35 percent more likely than those on the low-fat diet to reverse the condition and those on the Mediterranean diet supplemented with nuts were 28 percent more likely to reverse metabolic syndrome, according to the study.

Research results were published online Oct. 14 in CMAJ. The Spanish government funded the study.

In the study, Salas-Salvado's team looked at almost 6,000 men and women at risk for heart disease. At the start, almost two-thirds had metabolic syndrome, which increases the risk of heart disease, diabetes and death, according to the study.

After a follow-up period of about five years, 28 percent of those who had metabolic syndrome at the start did not have it, the study found. Those who ate the Mediterranean diet were more likely to reverse the condition, the researchers reported.

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Those who ate the Mediterranean diet also had a decrease in belly fat, which is known to increase heart disease risk, according to the study.

Many ingredients found in the Mediterranean diet can explain these effects, Salas-Salvado said. Monounsaturated fatty acids (MUFA), which are found in olive oil, replace saturated fatty acids in a Mediterranean diet, and this helps the cholesterol profile and insulin sensitivity (which decreases type 2 diabetes risk), according to the study.

Olive oil-rich diets have been shown to reduce blood pressure, he said. And most nuts are also rich in MUFAs, according to the U.S. Centers for Disease Control and Prevention.

The new findings add further proof that the Mediterranean diet "can play a role in overall health promotion, not just cardiovascular disease prevention," said Connie Diekman, director of university nutrition at Washington University in St. Louis.

This study also replicates other research that suggested that the diet could have an effect on belly fat, she said. ''What is important, and this has been observed in other studies, is that the Mediterranean diet can aid the reduction in central obesity, a key factor in development of metabolic syndrome," Diekman said.

It's definitely possible for people in the United States to adopt the Mediterranean style diet, Salas-Salvado said. The foods are at hand, but Americans may need to devote more time to preparation and cooking, he said, as well as taking time to eat the foods so they enjoy them.